1. Field of the Invention
The present invention relates to an electric power system comprising an AC motor, a first DC voltage unit, a power converter such as an inverter for exchanging the power between the aforementioned components, and an auxiliary controlled DC power supply that is connected between the AC motor and first DC voltage unit, and in particular to an electric power system enabling a control of the controlled DC power supply by means of a simple configuration.
2. Description of the Related Art
FIG. 1 is a configuration diagram of an electric power system for driving an alternate current (AC) motor by means of an inverter, showing the case of the three-phase inverter and AC motor.
Referring to FIG. 1, the numeral 10 is an inverter constituted by a semiconductor switching element 11, the numeral 100 is an AC motor, and the numeral 20 is a direct current (DC) voltage unit constituted by a voltage smoothing-use capacitor 21.
The electric power system is configured to make the inverter 10 convert a DC power of the DC voltage unit 20 into an AC power and to control a power supplied to the AC motor 100. In this case, the voltage of the DC voltage unit 20 is maintained approximately at constant, and a peak value of an AC output voltage of the inverter 10 is fundamentally equal to or less than the voltage value of the DC voltage unit 20.
In order to exchange a large power between the inverter 10 and AC motor 100 for rotating it in high speed, an increase of an AC voltage applicable to the AC motor 100 from the inverter 10 by increasing the voltage of the DC voltage unit 20 is effective. Here, when using a storage battery (called as “battery” hereinafter) 30 as a DC voltage unit as shown in the drawing, the voltage of the battery 30 must be increased for increasing the DC voltage, requiring the number of serially connected batteries to be increased and hence causing a cost increase.
Meanwhile, the voltage of a battery usually has a tens (10s) % variation range against a reference value depending on a charged condition, requiring a system design in response to the variation range. This means, for example, the design must determine the maximum current of the motor so that a prescribed output can be obtained even if the voltage of the battery is at the lowest.
This results in creating problems of increasing a cost, volume and size such as increasing a coil lead diameter of a motor for allowing a large current, requiring a cooling mechanism responsive to an increased heat generation associated with the current flow.
Now turning to FIG. 2 which is a configuration diagram of an electric power system that has added a voltage booster (i.e., a step-up chopper) to the configuration shown in FIG. 1, aiming at solving the various problems described above.
Referring to FIG. 2, the numeral 31 is a low voltage battery, the 51 numeral is a DC reactor of which a terminal is connected to the positive electrode of the battery 31 and the other terminal is connected to a connection point of a pair of semiconductor switching elements 41 within the voltage booster 40 which is also parallelly connected to the DC voltage unit 20.
The configuration makes it possible to step up a DC input voltage of the inverter 10 by an operation of the voltage booster 40 and maintain the voltage of the DC voltage unit 20 high even when a low voltage battery 31 is used. It is also possible to control the voltage of the DC voltage unit 20 at constant by virtue of a function of the voltage booster 40 even if the voltage of the battery 31 fluctuates.
The voltage booster 40, however, commonly requires the DC reactor 51, hence ushering in problems anew such as increasing cost, size and weight of the overall system.
Furthermore, FIG. 3 is a configuration diagram of an electric power system noted in a later described reference patent document 1. The electric power system is configured to connect a battery 32 between a neutral point of a coil of the AC motor 100 and either of the negative or positive pole (i.e., the negative pole in the configuration of FIG. 3) of the DC voltage unit 20.
An outline of the operation of the electric power system shown in FIG. 3 is as follows.
There is a mode of turning on or off all switching elements 11 of the upper arm simultaneously and a mode of turning on or off all switching elements 11 of the lower arm simultaneously in the inverter 10. There is a leakage inductance in the AC motor 100.
The switching elements 11 of the upper arm, which turn on or off simultaneously, and the switching elements 11 of the lower arm, which turn on or off simultaneously, can be regarded as one switching element, respectively, while the leakage inductance of the AC motor 100 can be regarded as DC reactor. FIG. 4 is a result of accordingly drawing an equivalent circuit of the circuit shown in FIG. 3, comprehensibly showing equivalency to the voltage booster 40 and the peripheral circuit thereof shown in FIG. 2 in terms of a circuit configuration.
Referring to FIG. 4, the numeral 10′ is equivalent to the inverter of FIG. 3 that is shown by one upper and lower arm by means of a zero-phase equivalency equivalent circuit, the numeral 101 is an equivalent to the leakage inductance of the AC motor 100, the numeral 321 is an ideal DC power supply equivalent to the battery 32 and the numeral 322 is an internal resistance of the battery 32.
A current flowing in the battery 32 can be regarded as so-called “zero-phase current” for the AC motor 100, fundamentally ineffective to a generated torque thereof. In fact, when the core of the AC motor 100 is magnetically saturated, a torque is changed because the magnetic flux of the motor influencing the torque is affected; the problem, however, can be alleviated by a design of the motor.
The electric power system shown in FIG. 3 is configured to control timings of turning on or off all of the switching elements 11 of the upper arm simultaneously, or all of the switching elements 11 of the lower arm simultaneously, thereby making it possible to control the power of the battery 32, that is, the voltage at a DC voltage unit of the inverter 10, without influencing the torque of the AC motor 100.
The major characteristic of the electric power system is, first, the leakage inductance equivalency 101 of the AC motor 100 performs a role of the DC reactor 51 in the configuration of FIG. 2, eliminating a necessity of the DC reactor 51, and furthermore, the switching elements 11 constituting the inverter 10 perform a role of the switching elements 41 of the voltage booster 40 in the configuration of FIG. 2, eliminating a necessity of an additional switching element as well, thereby enabling a simplification of a circuit configuration and a compact design of the entire system.
[Patent document 1] Registered Japanese Patent No. 3223842 (See paragraphs 0029, 0030, FIG. 10 in particular), which corresponds to U.S. Pat. Nos. 6,137,704 and 6,137,704. The latter US patent is a divisional application of the former US patent.
Here, in the case of attempting to utilize a voltage applied to the motor by the inverter 10′ to the maximum in the conventional technique shown in FIG. 4 (that is, FIG. 3), a situation arises in which a power of the battery 32 cannot be controlled due to a limitation of a DC power supply voltage of the battery 32, resulting in being unable to utilize a power of the inverter 10′ to its best. This can be explained as follows.
In order to control the power of the battery 32, an average voltage at the neutral point of the AC motor 100, to which the battery 32 is connected, relative to the negative pole of the DC voltage unit 20 needs to be approximately the same as the voltage of the battery 32 because an average current flowing therein is determined by the following expression 1:Idcs=(Vn−Vdcs)/Rdcs  [Expression 1]
where:
Idcs: average current value flowing in the battery 32
Vn: average voltage value at the neutral point of the AC motor 100 relative to the negative pole of the DC voltage unit 20
Vdcs: open voltage of the battery 32
Rdcs: internal resistance of the battery 32
That is, the Rdcs is generally small, making the (Vn−Vdcs) relatively a small value, and also it needs to be Vn>Vdcs if the battery is desired to be charged, while it needs Vn<Vdcs if a power is desired to be obtained by discharging the battery 32, and therefore the Vn needs to be controlled at “Vdcs plus or minus equivalency of adjustment”.
In the case of applying the maximum voltage to the AC motor 100 from the inverter 10′, that is, of a modulation ratio of the inverter 10′, being approximately “1”, the Vn ends up being fixed to approximately one half (=Edc/2) of the voltage Edc of the DC voltage unit 20, making “adjustment allowance” of the Vn almost disappear.
In such a case, the voltage of the battery 32 must be approximately Edc/2, making a space for adjusting the Vn disappear and resulting in being unable to control a power of the battery 32. This consequently makes it difficult to utilize the output of the inverter 10′ at its best. Such a situation degrades a freedom of design of an electric power system, hampering an implementation.